Catalytic materials, such as supported metal catalysts, are usually prepared by impregnating a porous high surface area support medium with a salt of the catalytic metal in solution or ion-exchanging atoms of the catalytic metal for certain of those of the support medium.
In the case of salt impregnation, the pores of the support medium are impregnated with sufficient solution to coat the outside of the support medium if a surface shell of metal around the outside of the catalyst particle is wanted or the support medium may be completely soaked in the metal salt solution, the support particles drained, dried, and calcined in air to convent the metal salt to the corresponding oxide. The oxide is reduced by hydrogen to the zero valent metal atoms at the sites of the oxide particles on the surface or within the pores of the particles of support medium.
In the ion exchange method, complexed catalyst metal atoms replace acidic hydrogen atoms of the support medium, the support is washed with deionized water to remove all free salt, the support dried, calcined, and reduced. Catalysts prepared as above have the metal dispersed on surface of the support, including inside the pores, as small crystallites of varying sizes.
Another type of catalytic material is porous metal-platable materials, used in electrochemical catalytic reactions such as fuel cells, electrochemical production or as electrodes for other reactions. Processes for chemical plating the surface of and within the pores of solid polymer materials have been used to make electrodes for fuel cells, for instance, by LeDuc, as shown in U.S. Pat. No. 3,235,473, where a continuous layer of silver was deposited from solution on the surface and within the pores of polyethylene. An electroplating process disclosed in U.S. Pat. No. 4,204,918 plated metal onto the walls of the void spaces in porous metal sheets, such as nickel, to provide porous catalytic materials.
The preferred materials for use as starting materials in the present invention and the process for making them are those disclosed in U.S. Pat. Nos. 4,557,957 and 4,720,400, issued to the present inventor and the two references are hereby incorporated by reference. The patents describe microporous polytetrafluoroethylene (PTFE) articles, having a microstructure of nodes interconnected by fibrils in which a continuous metal coating plates and incapsulates nodes and fibrils of the microporous PTFE structure, while maintaining substantially the original porosity of the unplated microporous PTFE from which the metal-plated articles were formed.